Preparation and Characterization of Epoxy/Polyhedral Oligomeric Silsesquioxane Hybrid Nanocomposites

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Oligomeric Silsesquioxane Hybrid Nanocomposites

JIEH-MING HUANG,1_{HUI-JU HUANG,}1_{YU-XIANG WANG,}1_{WEN-YI CHEN,}2_{FENG-CHIH CHANG}2 1_{Department of Chemical and Materials Engineering, Vanung University, 1, Van Nung Road, Chung-Li,} 32054, Taiwan, Republic of China

2

Institute of Applied Chemistry, National Chiao-Tung University, Hsin-Chu, Taiwan, Republic of China

Received 24 October 2008; revised 25 June 2009; accepted 29 June 2009 DOI: 10.1002/polb.21788

Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: We have prepared epoxy/polyhedral oligomeric silsesquioxane (POSS) nanocomposites by photopolymerization from octakis(glycidylsiloxy)octasilsesquiox-ane (OG) and diglycidyl ether of bisphenol A. We used nuclear magnetic resonance, Raman, and Fourier transform infrared spectroscopies to characterize the chemical structure of the synthetic OG. Differential scanning calorimetry and dynamic mechanical analysis (DMA) revealed that the nanocomposites possessed higher glass transition temperatures than that of the pristine epoxy resin. Furthermore, DMA indicated that all of the nanocomposites exhibited enhanced storage moduli in the rubbery state, a phenomenon that we ascribe to both the nano-reinforcement effect of the POSS cages and the additional degree of crosslinking that resulted from the reactions between the epoxy and OG units. Thermogravimetric analysis revealed that the thermal stability of the nanocomposites was better than that of the pristine epoxy.VVC2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1927–1934, 2009

Keywords: epoxy; nanocomposites; photopolymerization; POSS; thermal properties

INTRODUCTION

Hybrid materials possessing both inorganic and organic components are interesting substances because of their potentially increased perform-ance capabilities relative to those of either of their nonhybrid species. Recently, novel classes of organic/inorganic hybrid materials have been developed based on polyhedral oligomeric silses-quioxane (POSS),1–7 a cubic form of silica that is rigid, has deﬁned dimensions (0.53 nm), and presents eight organic groups (functional or inert) at the vertices of the cube. POSS derivatives have several advantages over conventional inorganic

ﬁllers, including monodispersity, low density, high thermal stability, and controllable functionalities. In addition, POSS monomers can be blended directly with polymers or copolymerized with other monomers to form polymer/POSS nanocom-posites. Dispersing an inorganic POSS component uniformly within an organic polymer matrix at the nanoscale level can have a synergistic effect

on improving the bulk properties.8–10 Several

nanocomposite hybrid polymers incorporating POSS exhibit increased values of their glass transition (Tg), and decomposition (Tdec) tempera-tures, improved resistance to oxidation, and

reduced ﬂammability.11–13 Incorporating

mono-functional or multimono-functional POSS-epoxy into the backbone of epoxy resin improves its thermal properties.14–18 Octakis(glycidyldimethylsiloxy)oc-tasilsesquioxane (OG) is the most-studied epoxy-Journal of Polymer Science: Part B: Polymer Physics, Vol. 47, 1927–1934 (2009)

V

VC2009 Wiley Periodicals, Inc.

Correspondence to: J.-M. Huang (E-mail: jiehming@mail. vnu.edu.tw)

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model systems for determining nanostructure-processing property relationships to demonstrate that nanoscale structural manipulation of the organic component can signiﬁcantly change mac-roscale physical properties.

UV-induced cationic photopolymerization of epoxy monomers is a well-known process and of great interest as a result of its high number of industrial applications. The UV-polymerizable formulations are solvent free, the production rates are high, and the energy required is much less compared to thermal curing. Besides, cationic photopolymerization of epoxy monomers have some distinct advantages, such as lack of inhibi-tion of oxygen, low shrinkage, and good mechani-cal properties of the UV-cured materials. The ini-tiation of cationic UV-curing polymerization is a multistep process involving ﬁrst, the photoexcita-tion of diaryliodonium or triarysulfonium salts, and then the decay of the resulting excited single state with both, heterelytic and homolytic clea-vages: cations and aryl-cations generated are very reactive with monomers to give the formation of a Bronsted acid, which is the actual initiator of cati-onic polymerization.24Initiation of polymerization takes place by protonation of the monomer, fol-lowed by the addition of further monomer mole-cules, thus resulting in chain growth reaction.25,26 In a previous study, we prepared a novel epoxy

nanocomposite possessing a low value of Dk by

introducing a ﬂuorine-containing POSS structure

into the photopolymerization system.27 In this

present study, we ﬁrst synthesized the OG mono-mer and then hybridized it with DGEBA and UVI 6976 to prepare a UV-cured epoxy resin. We expected the OG monomer units to be incorporated into the backbone of the epoxy resin upon UV cur-ing. Herein, we discuss the morphologies and ther-mal properties of the prepared nanocomposites. EXPERIMENTAL

Materials

The octakis(dimethylsilyloxy)silsesquioxane

(HMe2SiOSiO1.5)8 (Q8M8H) and platinum

1,3-divinyl-1,1,3,3-tetramethyldisiloxane [Pt(dvs)]

were purchased from Aldrich Chemical Co. (USA). The allyl glycidyl ether (AGE) was purchased from Acros (Belgium). The DGEBA

(EEW ¼ 188 g/eq.) was purchased from Nan Ya

Plastic Co. (Taiwan). The triarylsulfonium hexa-ﬂuoroantimonate (UVI 6976, photoinitiator) was purchased from Dow Chemical Co. (USA). The chemical structures of these compounds are provided in Scheme 1.

Octakis(dimethylsiloxypropylglycidyl ether)silsesquioxane (OG)19

A solution of Q8M8H(0.50 g, 0.49 mmol) in toluene (5 mL) was stirred magnetically for 5 min in a 25-mL Schlenk flask. AGE (0.46 mL, 3.92 mmol) was then added, followed by 2.0 mM Pt(dvs) (10 drops). The reaction mixture was stirred at 80C for 8 h, cooled, and then dry activated char-coal was added. After stirring for 10 min, the mix-ture was filtered through a 0.45-lm Teflon mem-brane into a vial and stored as a clear solution having a concentration of 10 wt %. The solvent was evaporated and an opaque viscous liquid was obtained (0.86 g, 90%). The synthetic reaction is presented in Scheme 2. The molecular weight of the OG POSS was 1818.9 g/mol.

Scheme 1. Chemical structures of Q8M8 H

, AGE, DGEBA, and UVI 6976.

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Epoxy/POSS Nanocomposites

Table 1 lists the codes and compositions used in this study. The compositions are based on weight ratio. The photoinitiator (UVI 6976) used in all the composition was 5 phr. In a typical process, photopolymerization was performed by placing the sample mixture onto a glass plate at a thick-ness of 300 lm. A 180-W medium-pressure arc

lamp (kmax¼ 366 nm) was then used to irradiate

the sample at a distance of 10 cm for 60 min at room temperature. The UV-cured sample was

then heated (postcured) at 160 C for 2 h. The

structure of the fully-cured epoxy/OG nanocompo-sites is shown in Scheme 3.

Characterization

Proton Nuclear Magnetic Resonance (1H NMR) Spectroscopy

1

H NMR spectra were recorded from CDCl3

solu-tions using a Bruker AMX-500 FT NMR spec-trometer operated at 500 MHz.

Fourier Transform Infrared (FTIR) Spectroscopy FTIR spectra were recorded on a Perkin–Elmer Spectra One infrared spectrometer; 32 scans were collected with a spectral resolution of 1 cm1. FTIR spectra of the epoxy hybrid films were recorded from samples prepared using conven-tional NaCl disk methods. The hybrid was cast onto a NaCl disk, which was dried under con-ditions similar to those used for the bulk pre-paration. The films obtained in this way were sufficiently thin to obey the Beer-Lambert law.

Raman Spectroscopy

Raman spectra of the epoxy/POSS ﬁlms were recorded using a Renishaw 2000 Raman spec-trometer equipped with a He-Ne laser (1 mW) to irradiate the sample at a wavelength of 785 nm

and a charge-coupled device (CCD) detector oper-ating at a resolution of 1 cm1.

Differential Scanning Calorimetry (DSC)

DSC measurements were performed under a nitrogen atmosphere using a Perkin–Elmer DSC-7 differential scanning calorimeter. Each sample (5 mg) was heated from 30 to 250C at a

heat-ing rate of 10 C/min. The Tg temperature was

taken as the midpoint of the capacity change. Thermogravimetric Analysis (TGA)

A Perkin–Elmer thermogravimetric analyzer

(TGA-7) was used to investigate the thermal sta-bility of the nanocomposites. The samples (5 mg) were heated under a nitrogen atmosphere from

ambient temperature to 800C at a heating rate

of 10 C/min. The thermal degradation

tempera-ture was taken as the onset temperatempera-ture at which a weight loss of 5 wt % occurred.

Dynamic Mechanical Analysis (DMA)

DMA measurements were performed using a TA Instruments DMA Q800 (DuPont) apparatus operated in a tensile ﬁlm mode over a

tempera-ture range from 30 to 250 C. Data acquisition

and analysis of the storage modulus (E0), loss

modulus (E00), and loss tangent (tan d) were

recorded automatically by the system. The heat-ing rate and frequency were ﬁxed at 3C/min and

Scheme 2. Synthesis of OG.

Table 1. Codes and Compositions Used in this Study

Code Composition

DGEBA/OG0 DGEBA:UVI 6976:OG¼ 100:5:0 (phr)

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1 Hz, respectively. The sample’s dimensions were 4 0.4 0.03 cm.

Scanning Electron Microscopy (SEM)

Samples of the epoxy/POSS nanocomposites were fractured cryogenically with liquid N2. The

cryo-genically fractured surfaces of the specimens were coated with thin layers of Pt and then their morphologies were examined under a Hitachi S-4700I scanning electron microscope.

RESULTS AND DISCUSSION Characterization of OG

Figure 1 presents the 1H NMR spectra of Q8M8 H and OG. The chemical shifts of the signals for the CH3and SiH groups of Q8M8Happear at 0.11 and 4.71 ppm, respectively.28 The chemical shifts of

the signals for the protons of the SiCH2CH2

groups of OG appear at 0.57 and 1.60 ppm; no sig-nal appears at 4.71 ppm, revealing that all of the SiH groups had reacted. Figure 2 displays the13C NMR spectrum of OG; the signal for the carbon

atoms of the CH3 groups appears at 0.5 ppm

and those for the epoxy groups appear at 50.6 and 44.0 ppm. Figure 3 provides the FTIR spectra of

Scheme 3. Formation of epoxy/OG nanocomposites.

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Q8M8H, AGE, and OG. In the FTIR spectrum of Q8M8H, we observe a sharp, strong, and symmet-ric peak at 1100 cm1corresponding to stretching

of the SiAOASi units of the silsequioxane cage

and a SiAH stretching peak at 2140 cm1.19,29,30

The presence of the signal at 1100 cm1 reveals

that the cube structure survived during process-ing; if it had degraded, we would observe a shift to the asymmetric broad peaks that are typical of

silica.31,32 In the FTIR spectrum of AGE, we

observe a small CH2¼¼CH stretching peak at

1640 cm1and an epoxy ring asymmetric

stretch-ing peak at 915 cm1. The signals of both the

SiAH and CH2¼¼CH units are absent from the

FTIR spectrum of OG, implying that hydrosilyla-tion had occurred successfully to form the POSS

derivative presenting epoxy rings. Figure 4 presents the Raman spectra of Q8M8

H

, AGE, and

OG. We assign the peaks at 2905 and 2140 cm1

to the SiACH3 and SiAH stretching modes,

respectively, of Q8M8 H

and the signal at 1651 cm1to the C¼¼C backbone stretching of AGE. As expected, the signals at 2140 and 1651 cm1were absent from the Raman spectrum of OG. Taken together, the FTIR, Raman, and NMR spectra conﬁrmed that Q8M8

H

did indeed react with AGE through hydrosilylation to form OG.

Glass Transition Temperature

We used DSC and DMA to determine the Tg

tem-peratures of the epoxy/POSS nanocomposites. Figure 5 displays the DSC thermograms obtained from epoxy/POSS nanocomposites containing var-ious contents of OG; Table 2 summarizes the results. A single value of Tgexisted for each of the

nanocomposites. The Tg temperatures of the

OG-Figure 3.FTIR spectra of (a) Q8M8H, (b) AGE, and (c) OG.

Figure 4. Raman spectra of (a) Q8M8H, (b) AGE, and (c) OG.

Figure 5. DSC thermograms of epoxy/POSS nano-composites containing OG contents of (a) 0, (b) 2, (c) 5, (d) 10, and (e) 20 phr.

Table 2. Glass Transition Temperatures and Heat Capacities of the Epoxy and Epoxy/POSS

Nanocomposites Code DSC DMA Tg(C) DCp (J/gC) Tg(C) tan d DGEBA/OG0 111.3 0.321 125.2 0.70 DGEBA/OG2 114.5 0.304 130.5 0.67 DGEBA/OG5 117.4 0.255 132.3 0.62 DGEBA/OG10 126.2 0.202 136.4 0.52 DGEBA/OG20 133.3 0.134 142.8 0.40

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containing epoxy nanocomposites were higher than that of the plain epoxy; in addition, the value of Tgincreased upon increasing the OG content. Viscoelastic Properties of the Epoxy/POSS Nanocomposites

Figure 6 displays the storage modulus (E0) of the pure epoxy and the epoxy/POSS nanocomposites recorded over the temperature range from 50

to 225 C. For the epoxy resin, the modulus

decreased continuously upon increasing the tem-perature above its Tgtemperature. The values of E0values of the hybrids were all higher than that of the pure epoxy. In addition, the values of E0 of the nanocomposites containing greater OG con-tents remains more of less constant at higher tem-peratures. In the rubber state, we found that the epoxy/POSS nanocomposites possessed higher

values of Tgwhen they contained higher OG

con-tents. We attribute this behavior to a signiﬁcant nano-reinforcement effect of the POSS cages on the epoxy networks. Because the nanometer-scale POSS cages restrict the motion of the macromo-lecular chains, higher temperatures are required to provide the requisite thermal energy for the Tg to occur in the hybrid materials.14 Furthermore, because the OG monomers reacted with the epoxy units, their presence increased the crosslink den-sity relative to that in the epoxy-only matrix. The plot of tan d versus temperature for a cured epoxy provides the major relaxation transition,

corre-sponding to the value of Tg of the cured epoxy

resin. Figure 7 provides the tan d plot of the DMA

an OG content of 20 phr was 142.8 C,

signiﬁ-cantly higher than that of the neat epoxy. Again,

this behavior reﬂects the signiﬁcant

nano-reinforcement effect of the POSS cages and the increased crosslink density that existed after the reaction between epoxy and OG; i.e., the incorpo-ration of POSS cages into the epoxy network hin-ders the movement of the polymer chains and, thereby, results in higher values of Tg. In addi-tion, Figure 7 also reveals a slight depression in the peak intensity upon increasing the OG con-tent. Because the damping properties are pro-vided by the ratio of the viscous and elastic com-ponents, we surmise that a reduced peak height is associated with a lower segmental mobility and fewer relaxation species and, therefore, it is indic-ative of stronger bonding for the epoxy/POSS nanocomposites. Moreover, the width of the peak in the tan d plot reﬂects the degree of structural homogeneity in a crosslinked network; for our sys-tem, the peak width at half height increased upon increasing the OG content—a result of decreasing network homogeneity.19,33Presumably, the broad-ening of the Tgregion resulted from the incorpo-ration of the relatively bulky (nanometer-sized) POSS cages, which restricted the segmental

Figure 6. Storage moduli of epoxy/POSS nanocom-posites containing OG contents of (a) 0, (b) 2, (c) 5, (d) 10, and (e) 20 phr.

Figure 7. Plots of tan d of the epoxy/POSS nano-composites containing OG contents of (a) 0, (b) 2, (c) 5, (d) 10, and (e) 20 phr.

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motion of the polymer chains and network

junc-tions. Therefore, higher temperatures were

required to reach structural equilibrium.34

Morphology of the Epoxy/POSS Nanocomposites Figure 8 presents SEM micrographs of the frac-tured surfaces of epoxy/POSS nanocomposite specimens created by freezing in liquid N2. The particle size of the OG units in the epoxy/POSS

(2 phr OG) nanocomposite was10 to 30 nm; it

increased upon increasing the OG content in the nanocomposites. For the epoxy/POSS (10 phr OG) nanocomposite, the particle size of the OG units

was20 to 60 nm because of the immiscibility of

OG and the cured epoxy.

Thermal Stability of the Epoxy/POSS Nanocomposites

Figure 9 and Table 3 provide TGA thermograms

and data, respectively, recorded under a N2

atmosphere for the epoxy resins containing vari-ous OG contents. The 5 wt % weight loss tempera-ture (T5) increased upon increasing the OG con-tent up to 5 wt %, but decreased thereafter, presumably because of excess aggregation at higher OG contents. In contrast, the char yields of the epoxy/POSS nanocomposites increased upon increasing the OG content. This result is not surprising because when POSS structures are

degraded thermally they typically form SiO2,

which results in higher char yields.30

CONCLUSIONS

We have prepared epoxy/POSS nanocomposites through photopolymerization of OG and the diglycidyl ether of bisphenol A, and characterized them using NMR, Raman, and FTIR

spectroscop-ies. DSC analysis indicated that a single Tg

occurred for each nanocomposite, with the value

of Tg increasing upon increasing the content of

OG. DMA revealed that the nanocomposites exhibited enhanced storage moduli in the rubbery

state, which we ascribe to (i) the

nano-Figure 8. SEM micrographs of epoxy/POSS nanocom-posites containing OG contents of (a) 2 and (b) 10 phr.

Figure 9. TGA thermograms of epoxy/POSS nano-composites containing OG contents of (a) 0, (b) 2, (c) 5, (d) 10, and (e) 20 phr.

Table 3. Thermal Properties of the Epoxy and Epoxy/POSS Nanocomposites Code T5(C, 5 wt % loss) Char Yield (wt %) at 800C DGEBA/OG0 380.4 6.05 DGEBA/OG2 384.5 12.92 DGEBA/OG5 394.3 18.91 DGEBA/OG10 380.8 23.74 DGEBA/OG20 375.3 28.65

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The authors thank the National Science Council (Tai-wan, Republic of China) for supporting this research ﬁnancially under contract no. NSC-94-2216-E-238-003.

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